CD44 is a broadly expressed cell surface molecule of which there are at least two isoforms. The first is an 80-9OkD glycoprotein expressed in hematopoietic cells and also in fibroblasts, neural cells and some epithelial cell lines. This form, which has recently been shown to bind hyaluronate, will be referred to as CD44H. The second isoform is a l5OkD glycoprotein expressed in a subpopulation of epithelial cells and will be referred to as CD44E. The ligands of CD44E are presently unknown. CD44H has been shown to mediate cell-cell and cell-substrate adhesion and has therefore been implicated in several sectors of biology, including the immune response, cellular growth, tumor development and metastasis. In the work, we propose to study the function and biology of both CD44 isoforms. Our efforts will be concentrated on elucidating the role of CD44H and CD44E in tumor growth in vivo and determination of patterns of tumor migration. Recently isolated cDNA clones encoding both isoforms will be used to create stable transfectants in human and murine cell lines. The transfectants in human and murine cell lines. The transfectants will be injected into nude mice and assessed for tumorigenic potential. Stable transfectants expressing either of the two isoforms with truncated cytoplasmic domains will be used to determine the role of signal transduction in CD44-mediated tumorigenesis as well as in physiologic cellular functions. Soluble forms of both CD44H and CD44E will be created and used to identify and isolate ligands specific for each isoform and to block transfectant-induced tumor growth and metastasis. Generation of a battery of monoclonal antibodies against both isoforms will allow the determination of their tissue distribution in adult and developing tissues, epitope mapping and identification of ligand binding sites. Site specific mutagenesis of the extracellular domain of CD44H will determine amino acid sequences required for interaction with hyaluronate. It is anticipated that these approaches will elucidate the physiologic function of the two isoforms as well as their influence on tumor growth. Understanding of the role of these molecules in tumor growth regulation may have important clinical implications.